When an output impedance of a transmission circuit or an input impedance of a reception circuit and a characteristic impedance of a transmission line are mismatched, a signal reflection occurs due to the impedance mismatch. This signal reflection causes a distortion (or deterioration) in the signal waveform on the transmission line, and increases the error rate of the signals that are transmitted.
For this reason, in high-speed signal transmission systems, the connection format has changed from the multi-drop bus connection which easily generates the impedance mismatch to the point-to-point connection which provides appropriate termination. However, in signal transmission systems which operate at even high speeds, the impedance mismatch is generated due to the signal transmission lines on the LSI package or board, through holes, connectors and the like, and the signal waveform is deteriorated by the signal reflection caused by the impedance mismatch. On the other hand, if an attempt is made to suppress the impedance mismatch by improving the fabrication accuracy of the signal transmission lines on the LSI package or board, through holes, connectors and the like, both the number of fabrication stages and the fabrication cost will increase.
Accordingly, in high-speed signal transmission systems, there are demands to reduce the effects of the signal reflection caused by the impedance mismatch by a relatively simple solution.
A Japanese Laid-Open Patent Publication No. 2004-363861 proposes an example of a signal transmission system which reduces the effects of the signal reflection caused by the impedance mismatch. A transmitting end circuit block includes a level monitor circuit to detect an intensity of the reflected wave. Information related to the reflected wave is extracted and sent to a receiving end circuit block via a signal wiring provided separately from a main signal wiring, and a waveform-shaping (or reshaping) is performed by subtracting the reflected wave component from the received signal in the receiving end circuit block. Alternatively, instead of subtracting the reflected wave component from the receiving signal in the receiving end circuit block, the reflected wave component is subtracted in advance from the transmitting signal in the transmitting end circuit block. The information related to the reflected wave is extracted by sending a unit pulse from the transmitting end circuit block prior to sending the transmitting signal, and detecting the reflected wave of the unit pulse by the transmitting end circuit block.
The circuit construction becomes complex and the circuit scale becomes large if a timer circuit and the level monitor circuit are formed using an Analog-to-Digital Converter (ADC) as proposed in the Japanese Laid-Open Patent Publication No. 2004-363861. Because the timer circuit and the level monitor circuit are connected to an output side of the transmitting end circuit block, an impedance mismatch is generated due to input capacitances of the timer circuit and the level monitor circuit, to thereby not only increase the reflection but also deteriorate the signal band. Particularly, in the case of the high-speed signals of 1 Gbps or higher, for example, the level monitor circuit required in the Japanese Laid-Open Patent Publication No. 2004-363861 is a flash type ADS which has an extremely large input capacitance that is proportional to 2N−1 if the number of output bits of the ADC is N.
In the Japanese Laid-Open Patent Publication No. 2004-363861, the information related to the reflected wave is extracted by sending the unit pulse from the transmitting end circuit block prior to sending the transmission signal and detecting the reflected wave of the unit pulse by the receiving end circuit block. For this reason, the reflected wave component may not be extracted while sending the transmitting signal from the transmitting end circuit block or, while receiving the signal by the receiving end circuit block. Accordingly, with respect to a dynamic change of the transmission line, such as a temperature change and deterioration with time (or aging), it is impossible to always perform the extraction of the reflected wave component, the transmission of the reflected wave or, the subtraction from the received signal in an optimum manner.